Project 5: Project Summary/Abstract Myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML) arising from MDS (AL-MDS) are blood disorders that primarily affect the elderly, and negatively impact quality of life and life-span. Allogeneic hematopoietic cell transplantation (HCT) is the only proven cure for MDS and secondary AML (MDS/AL-MDS). However, long-term survival is limited by toxicity HCT and high relapse rates. These diseases arise from primitive clonal hematopoietic stem cells (HSC) that out complete normal HSC. Hence, the efficacy of HCT derives from the combination of eradication of disease-specific clones, replacement of MDS HSC with donor HSC, and an allogeneic graft-vs-leukemia (GVL) clone effect. CD117 is a cytokine receptor expressed on normal human HSC. Preclinical studies show that monoclonal antibodies (mAb) targeting CD117 can deplete endogenous HSC and progenitor cells, creating niche space that permit donor HSC engraftment. Thus, mAbs targeting CD117 have the potential to replace or augment the myeloablative component of HCT conditioning. To study this possibility, we translated the use of a humanized anti-human CD117 mAb, designated AMG 191, to a HCT trial wherein AMG 191 is the sole conditioning agent for children with the non-malignant disease, severe combined immunodeficiency (SCID). Here we show early efficacy data on AMG 191 activity these patients. We have also found that CD117 is expressed at high levels on disease initiating HSC in patients with MDS/AL-MDS, and that AMG 191 depletes MDS HSC in mice stably xenografted with primary human MDS cells. These data lead us to hypothesize that anti-CD117 mAbs can be used to deplete MDS/AL-MDS clones. The overall aim of this project is to determine if anti-CD117 biologic agents will safely eliminate MDS/AL-MDS HSC and enhance donor HSC engraftment leading to better HCT outcomes for these disorders. Aim 1 will test if anti-CD117 biologic agents can eradicate advanced stage human MDS/AL-MDS in xenografted mice, and permit durable engraftment of normal human HSC. Mice will be transplanted with purified human HSC. Hence no GVL or graft-vs-host disease (GVHD) is expected. Aim 2 will test if MDS/AL-MDS disease eradication can be potentiated in xenografted mice conditioned with anti-CD117 reagents by use of T cell replete grafts without GVHD. Grafts will contain HSC + T cells plus/minus regulatory cells described in Projects 1 and 2. The aim will also test if HSC plus CD8+ memory cells (Project 3) will enhance MDS/AL-MDS clone elimination without GVHD. Aim 3 proposes a Phase I/II clinical trial to test the safety and efficacy of AMG 191 added to our existing non-myeloablative regimen of total lymphoid irradiation (TLI) and anti-thymocyte globulin (ATG). We hypothesize that AMG 191 + TLI/ATG will facilitate full donor myeloid chimerism and thereby improve outcomes of MDS/AL-MDS patients. This clinical study is currently under review at the FDA. Our long-term goal is to establish a new therapeutic platform using anti- CD117 agents that will be applicable to most all diseases for which HCT is indicated.